In today's digital transmission networks, the technology of synchronization has been advanced to such a degree that a communication network is synchronized with faster transmission apparatuses employing optical transmission. For functions and configurations of the digital transmission networks and transmission apparatuses, worldwide standards have been established such that a transmission apparatus and/or a communication network may be introduced in conformity to the standards to provide high quality transmission anywhere in the world. Examples of specific standards may include the standard (established in 1988) on a transmission system referred to as “SDH” (Synchronous Digital Hierarchy) defined in Recommendation G.707 and so on by International Telecommunication Union-Telecommunication Sector (“ITU-T”), and the standard (established in 1991) on a transmission system referred to as “SONET” (Synchronous Optical Network) defined in Standard T1.105 American National Standard Institute (“ANSI”), both of which define the configuration of optical synchronous communication systems and functions of transmission apparatuses.
The SDH and SONET standards are intended for the process (for transmission or multiplexing/demultiplexing) of synchronous multiplex signals (frames) which are the main signal section of digitized and multiplex main signals called “payload” added by signals called “transport overhead” used for the operation, administration, maintenance and provisioning (OAM&P) of the transmission equipment and communication network. The transport overhead has a pointer, which is used for the stuff control of phase accommodation and frequency justification. Based on this scheme, it becomes possible to provide a transmission system which is less susceptible to transmission delay and is superior in the OAM&P ability.
Transmission techniques based on this kind of transport overhead are disclosed in Japanese Published Unexamined Patent Application No. Hei 4-79628 and U.S. Pat. No. 5,682,257 for example. In regard to the transmission scheme with the intention of enhancing the latitude of SDH or SONET-based network organization, there is known suggestive article T1X1.5/96-085 addressed to ANSI for example.
The synchronous digital transmission network of this type uses the bidirectional line switched ring (BLSR) which is the ring-wise connection of OC-12 transmission paths for example. The transmission paths have their protection switching made conformable to the protocols of the Bidirectional Line Switched Ring stated in the ANSI Recommendation T1.105.01 and the MS Shared Protection Rings stated in the ITU-T Recommension G. 841. Specifically, the protection switching operation is carried out among plural multiplex units which constitute the ring network by use of the K1 byte and K2 byte, which are called “automatic protection switching (APS) bytes”, placed in the transport overhead of synchronous-multiplex signals.
For grading up a transmission network, OC-12 paths in ring-wise connection are replaced partially with OC-192 paths which have a larger transmission capability. In this case, transaction of the above-mentioned K1 and K2 bytes needed for the protection switching operation is shut off at the multiplex transmission unit located between the OC-12 path and OC-192 path, causing the OC-12 ring network to fail to retain its protection switching operation. The reason is because the APS bytes sent over the OC-12 path is terminated by the multiplex unit connected at one end of the OC-192 path, and is not propagated to the counterpart of another end, as stated in the SONET and SDH standards. The APS bytes sent over the OC-192 path is used solely for the protection switching operation of the OC-192 path. Therefore, it is not easy in general to accomplish a network in which a section of the BLSR (Bidirectional Line Switched Ring) network of OC-M is multiplex to a network of OC-N (N is greater than M) having a larger transmission capability.
The sole feasible manner for this accomplishment is to send by through-transport the OAM&P information inclusive of the APS bytes, which comes in from the OC-12 path, through the OC-192 section. For example, it is assumed that multiplex units A, F, G, D and E form a BLSR network of A ⇄F ⇄G ⇄D ⇄E ⇄A, of which the F ⇄G is a high-speed OC-192 path section and the rest are low-speed OC-12 path sections. If a fault arises in the OC-12 path section E ⇄A, the multiplex units A and E detect the fault and transact the APS bytes which are coded in accordance with the BLSR protocol over the paths A ⇄F ⇄G ⇄D ⇄E by transporting through the OC-192 path section E ⇄G, thereby implementing the protectionswitching. This scheme allows a OC-12 ring network, even though it includes a OC-192 path, to retain the protection switching operation, and enables the network organization at a relatively high latitude.
However, the above-mentioned scheme can possibly fail to implement the protection switching in need in case a fault arises at a specific position of transmission paths. For example, if a fault arises in the transmission path section A ⇄F of the above-mentioned BLSR network, it is expected according to the above-mentioned scheme that the OAM&P information inclusive of the APS bytes from the OC-12 path should be sent by through-transport through the OC-192 path section and protection switching should be carried out. However, it does not take place this time. This fault differs from the above-mentioned case in that the faulty OC-12 path has its one end connected with the multiplex unit F which is connected to the OC-192 path. In such a case, even if the fault of the upstream path is indicated to the downstream multiplex unit D, BLSR switching (span switching or ring switching of BLSR) does not take place in the multiplex unit D. The following will explain the reason in detail.
In the SONET and SDH standards, it is stated that in response to the detection by a multiplex unit of a fault of transmission path such as the loss of signal, loss of frame, or AIS-L (or MS-AIS in the SDH), AIS-P (or AU-AIS in the SDH) which is the alarm for the STS path layer is transported to the downstream unit. The AIS-P is to set a “1”s bit string to the STS synchronous payload envelope and STS pointers (H1, H2 and H3 bytes). The OC-12 receiver of the unit F detects the loss of signal and transports the AIS-P to the downstream unit G. On receiving the AIS-P, the unit G further transports the AIS-P to the downstream unit D. However, in the SONET and SDH standards, the reception of AIS-P does not cause the BLSR switching of OC-12 paths.
Accordingly, any alarm information, either for transport overhead or payload, which causes the BLSR switching will not be transported depending on the faulty section, and therefore the BLSR switching of OC-12 paths which is needed for the unit D for example will not be implemented. Consequently, the OC-12 network is left unrecoverable. This signifies that the path which runs between the multiplex units A and D by way of the units F and G, for example, is left in the defective state, causing the OC-12 network to be inoperative.
A conceivable preventive manner against this impropriety is the multiplex units F and G installing the protection switching, i.e., BLSR switching, function for the OC-12 network, so that the units F and G are treated as nodes of the OC-12 network equally to the A and other multiplex units. Accordingly, the units F and G implement the BLSR switching of OC-12 paths in accordance with the APS bytes of the transmission frame of OC-12. At that time, the APS bytes for the BLSR switching of the OC-12 paths is sent through the OC-192 path section between the units F and G by being inserted into the undefined area of the line overhead of the OC-192 transmission frame. The BLSR switching is implemented by the unit G instead of F since a partial band of the OC-192 path cannot be operated for the OC-12 BLSR. This manner, however, necessitates the BLSR switching function for a maximum of 16 low-speed OC-12 paths, and it is not realistic from the viewpoints of system scale and cost.
The foregoing is an example of a fault occurring on the OC-12 path of the direction from unit A to unit F, and it is also relevant to a fault occurring on the OC-12 path of the direction from unit D to unit G. Moreover, in the event of a fault which is irrecoverable by the protection switching, e.g., simultaneous switching failure of the working line and protection line, on the OC-192 path between the units F and G, which is the case of demand of BLSR switching by the units A and D, switching does not take place by the same reason as described above.
A problem involved in the art stated above is that at the occurrence of a fault on a OC-M transmission path which is immediately preceding the multiplexing to a high-speed OC-N signal (N is greater than M), or at the occurrence of a fault which is irrecoverable by the protection switching on the OC-12 path, the protection switching which is inherently the case of demand by the OC-M network is not implemented and moreover the system is left in a state in which the OC-M signal is treated to be normal.